What Is An Unloading Check Valve?

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A visitor asks the question “what is an unloading check valve” as it pertains to air compressors.

These are one of the devices companies have manufactured to emulate the unloader valves that are commonly attached to, or part of, the air compressor pressure switch.

We expect you know that the unloader valve operates when the air compressor reaches cut out and stops. It opens to allow any air that might have been trapped over the compressor piston to escape to atmosphere. Otherwise, that trapped air may add so much load to the compressor motor when the compressor tries to start that it results in a blown fuse or circuit breaker.

But, what if the air compressor has a different sort of pressure switch? One that has no allowance for the addition of an unloader valve. This can be addressed in a couple of ways.

Hole in top of compressor piston

Some manufacturers drill a small hole in the top of the compressor piston. This means that all the while the compressor is running, pumping air into the tank, some of that compressed air is bleeding out the hole.

However, this same hole allows the air to bleed off the top of the piston when the air compressor stops, effectively emulating the operation of an unloader valve. Once the air over the compressor piston is gone, the air leak stops, unless your compressor has a faulty tank check valve, in which case, all the air in the tank will bleed out as well!

What Is An Unloading Check Valve?

The other method of emulating a compressor switch unloader valve is the unloading check valve.

Compressor unloading check valve - www.fix-my-compressor.com
Unloading check valve photo: www.cdivalve.com

Either installed in the tank where the line from the pump enters it, or in the line between the pump and the tank, the unloading check valve blocks its air-escape hole while pressurized air is passing through it from the compressor pump, as when the compressor is filling the tank.

When the tank is full and the pressure level triggers the pressure switch to off, the pressurized air flow through the unloading check valve stops, the small bleed hole in the unloading check valve opens, and that opening allows all the air in the line from the pump head to the tank to escape, effectively unloading the piston.

A quick search on your computer browser for unloading check valve will bring you a variety of sources.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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